1. Field of the Invention
The present invention relates to a signal processor and, more particularly, to a signal processor which amplifies a fixed frequency component of a signal when the signal is transmitted.
2. Description of the Related Art
Conventionally, a recording and reproducing apparatus is known as an apparatus for transmitting signals.
In a known magnetic recording and reproducing apparatus, a method of emphasis is adopted for the purpose of attenuating a noise component occurring in the signal.
This emphasis is a method whereby, when, for instance, a video signal is subjected to frequency modulation to effect magnetic recording, if only the deviation of the frequency modulation is expanded to reduce the noise component, an adverse effect such as moire caused by the signal of the sideband results, so that only the deviation of a high-frequency component of the input video signal is expanded to effect recording, thereby reducing the effect of the noise component during recording.
It is generally known that, by virtue of this emphasis, the spectrum of the noise component after frequency demodulation and demodulation shown in FIG. 1(a) is, after demodulation, converted into a spectrum such as the one shown in FIG. 1(b), thereby reducing the noise component.
In addition, methods called fixed emphasis and dynamic emphasis are known as methods of the above-described emphasis. A description will be given hereafter of these methods by using a video signal frequency modulation magnetic recording apparatus to which these methods are applied.
FIG. 2 shows a schematic diagram of a magnetic recording apparatus in which fixed emphasis is employed. In FIG. 2, the magnetic recording apparatus comprises a fixed emphasis circuit 200, a limiter circuit 201, a frequency modulation circuit 202, a recording amplifier 203, a magnetic head 204, and a magnetic sheet 205 constituting a magnetic recording medium. The fixed emphasis circuit 200 normally has amplification characteristics, as shown in FIG. 3(a), and is configured by a very simple circuit, as shown in FIG. 3(b). Since the configuration shown in FIG. 3(b) is generally known, a description thereof will be omitted.
In FIG. 2, the high-frequency component of an input video signal is amplified by the fixed emphasis circuit 200 on the basis of the amplification characteristics shown in FIG. 3(a). As a result, at the sharp rising and falling of the video signal, sharp spikes called white peak and dark peak are produced, as shown in FIG. 2, and these spikes cause the known inverted white peak and the like. Therefore, after the level at the portion of the spikes is shaped by the limiter 201, the video signal is subjected to frequency modulation by the frequency modulation circuit 202. At this time, with respect to the frequency-modulated signal output by the frequency modulation circuit 202, since the signal whose high-frequency component has been amplified by the fixed emphasis circuit 200 is subjected to frequency modulation, the deviation in the high-frequency component is modulated more than that of the low-frequency component.
Subsequently, the signal modulated as described above is amplified by the recording amplifier 203, and is then recorded by the magnetic head 204 on the magnetic sheet 205 which is being rotated by a motor (not shown).
In addition, FIG. 4 shows a schematic diagram of a magnetic recording apparatus in which dynamic emphasis is employed. In FIG. 4, the magnetic recording apparatus comprises a dynamic emphasis circuit 400, a limiter circuit 401, a frequency modulation circuit 402, a recording amplifier 403, a magnetic head 404, and a magnetic sheet 405 constituting a magnetic recording medium. The dynamic emphasis circuit 400 has characteristics in which the amplification factor changes non-linearly in response to the level of the input signal, and it is possible to obtain a greater effect of noise reduction as compared with the apparatus using the aforementioned fixed emphasis circuit 200. In addition, FIG. 5(a) shows the amplification characteristics of the dynamic emphasis circuit 400, while FIG. 5(b) shows a specific example of its configuration. Since the configuration shown in FIG. 5(b) is generally known, a description thereof will be omitted.
In FIG. 4, the high-frequency component of the input video signal is amplified by the dynamic emphasis circuit 400 on the basis of the amplification characteristics shown in FIG. 5(a). At the rising and falling of the video signal, the white peak and the dark peak occur as in the case of the afforementioend fixed emphasis circuit 200. However, the lower the level, the video signal is amplified with the higher amplification factor, so that appropriate amplification is effected in correspondence with the level of the input video signal.
After it is subjected to dynamic emphasis and its portions of the white peak and the dark peak are shaped by the limiter circuit 401, the video signal is subjected to frequency modulation by the frequency modulation circuit 402, goes through the recording amplifier 403, and is recorded on the magnetic sheet 405 being rotated by a motor (not shown) by means of the magnetic head 404.
Thus, the noise component of the high-frequency component of the recorded signal is reduced, as shown in FIG. 1(b), by a recording apparatus having the above-described emphasis circuit, while the recorded signal can be reproduced easily by a reproducing apparatus such as the one shown in FIG. 6.
In FIG. 6, the reproducing apparatus comprises a magnetic sheet 600 constituting a magnetic recording medium, a magnetic head 601, a pre-amplifier 602, a frequency demodulation circuit 603, and a de-emphasis circuit 604. After the signal reproduced from the magnetic sheet 600 by the magnetic head 601 is amplified by the pre-amplifier 602, the signal is demodulated by the frequency demodulation circuit 603 and the video signal is restored and output by the de-emphasis circuit 604 having opposite characteristics to those of the fixed emphasis circuit 200 shown in FIG. 2 or the dynamic emphasis circuit 400 shown in FIG. 4.
When the noise component is reduced by using the above-described emphasis circuit, the greater the amplification factor of the high-frequency component of the signal, the more the deviation in the high-frequency component expands during frequency modulation. As a result, the effect of reduction of the noise component becomes high.
However, if the amplification factor is made excessively high with respect to the high-frequency component, the known inverted white peak occurs during the frequency demodulation on the reproducing side, so that it becomes impossible to restore the signal properly.
The inverted white peak is a phenomenon in which, if a signal having a sharp peak produced by being subjected to an emphasis circuit is frequency-modulated and recorded, a zero cross point at a portion corresponding to the peak of the reproduced signal drops out during reproduction, and, if the recorded signal is a video signal, the signal which essentially represents white is restored as a signal representing black, or vice versa. To prevent the occurrence of this phenomenon, after emphasis is carried out, as described before, a limiter is applied by the limiter circuit by limiting the level of the peak portions to some extent.
Howver, if the above-described limiter is applied excessively, the known phenomenon of smear occurs, resulting in the deterioration of the reproduced signal, so that there is a limit to increasing the amplification factor in the fixed emphasis.
In addition, dynamic emphasis has been such that it is impossible to increase the amplification factor above a certain level since the non-linear characteristics cannot be set severely in consideration of the need to maintain the compatibility of the apparatus.
Particularly recently, there has been a strong demand for high-quality recording and reproduction of the video signal. Hence, the so-called high-band recording in which the video signal is frequency modulated and recorded by using a high-frequency carrier signal. Although it is necessary to increase the amplification factor in emphasis so as to reduce the noise component, conventional methods of emphasis make it difficult to increase the amplification factor, as described above. Hence, it has been impossible to effect a sufficient reduction of the noise component.